In general, tilt sensors can be classified into two kinds. One is the single axis tilt sensor, and the other is the dual axis tilt sensor. The single axis tilt sensor measures the slant of one direction, whereas the dual one estimates the slant of a plane.
Tilt sensors have a variety of applications due to their accurate measurements in level evaluating. Tilt sensors also have good reliability and are easy to obtain, so they are originally designed for the aim of weapons, the navigation of aircraft, and now the alignment of automobile wheels, even the detection of earthquakes and the applications of laser tilt meters. The principle of how the tilt sensor works is illustrated in FIG. 1. When the surface of the earth is not in horizontal status, the tilt sensor tilts, and the surface of the electrolyte in the tilt sensor remains horizontal status due to gravity. The electrolyte is electrically conductive, and the conductivity between the two electrodes of the tilt sensor is proportional to the length of that the electrode is immersed in the electrolyte. Thus, the resistance between the first and the second electrode and the resistance between the second and the third electrode are both changed in proportion to the tilt angle. Therefore, the slant of the tilt sensor is derived from the resistance corresponding to the tilt angle.
FIG. 2 illustrates a bottom view of a dual axis tilt sensor. Four outer electrodes A, B, C, and D are placed crisscross as signal inputs, and the central electrode E as signal output.
Please refer to FIG. 3 illustrating the input method of a conventional dual axis tilt sensor. The input method of the conventional dual axis tilt sensor is unidirectional, that is, one axis is excited at a time. Outer electrode C and outer electrode D of the sensor 301 are grounded. And outer electrode B of the sensor 301 is also first disconnected and then a symmetric square waveform is inputted into outer electrode A as the input signal. To prevent the electrolyte from being polarized, the DC components of the input signal must be zero. Hence, the input signal should be a symmetric square waveform. When it is not in horizontal status, the dual axis tilt sensor tilts, and the conductivity between the two electrodes is proportional to the length of electrode immersed in the electrolyte. Thus, the resistance between the two electrodes is changed in proportion to the tilt angle, thus the input signal is then reduced or amplified due to the variation of resistance, so as to acquire an asymmetric output signal from the central electrode E. By analyzing the voltage of the output signal, the level of the direction in outer electrode A and grounded outer electrode C will be gotten. After that, outer electrode A is disconnected and then a symmetric square waveform is input into outer electrode B as the input signal. These steps are repeated and the output signal is analyzed in the same way. The level of the other direction in outer electrode B and grounded outer electrode D can be gotten afterwards.
Since the input signals must be switched frequently and time sequence is also needed to access the signals correctly by this method, thus the signals cannot be sampled in real time. Otherwise, due to the serious interference between each input signal, the sensitivity is low and the fluctuation of the signals are great, thus the conventional method can not arrive the precise measurement. Furthermore, since a negative voltage for input signals is needed, a negative voltage supplier with high stability should be used. Otherwise, the switching of positive voltage and negative voltage also results in high complexity in circuit design, and it also increases the difficulty of the circuit design.